Agricultural chemical manufacturers are always looking for ways to improve the efficacy of active ingredients used on plants. This is particularly true where the applied material is a plant growth regulator or systemic agent (e.g., insecticide, fungicide, or herbicide). Transport mechanisms into the plant and translocation among the various plant tissues is important and, in some instances, may be the primary factor determining the efficacy of the applied ingredient.
It would be useful to have a means for increasing the uptake of agriculturally active ingredients into plants. It would be particularly helpful to have such a means for plant growth regulating agents and active ingredients operating systemically in plants.
Mepiquat chloride is one chemical in particular where plant uptake and transport is important. Mepiquat chloride (N,N-dimethylpiperidinium chloride) is used annually as the active ingredient for controlling excessive vegetative growth and increasing fruit retention on millions of acres of cotton with some uses on potatoes, sweet potatoes, peanuts, grapes, corn, wheat, citrus, tomatoes, and onions. The growth stunting effects are particularly desired when the crop is grown in fertile soil or after weather conditions that favor plant growth rather than fruit production.
Cotton is a representative plant for describing the uses and effects of mepiquat chloride. Mepiquat chloride has the effect on cotton plants of stunting vegetative growth thereby forcing the plant to redirect its energies into fruit (cotton boll) production. With appropriate application of mepiquat chloride to plants that are beginning to exhibit excessive vegetative growth, cotton plant yields can be maintained or increased without harm to the plant.
Cotton plants have a predictable life cycle and growth period. Cotton plants emerge 7-10 days after the seeds are planted in a furrow. The cotton plant exhibits growth of a root system and the extension of plant height through stem and branch growth in a pattern referred to as "vegetative growth" until about the 4th-8th node. Thereafter, the plant produces a reproductive branch (the "1st fruiting site"), and all subsequent branches are reproductive. Cotton growers attempt to control the growth of the plant to ensure that the ratio of vegetative growth to reproductive growth (boll production) favors the desired range of reproductive growth.
Cotton growers generally prefer to see about 2 inches (5 cm) between main stem nodes. This ratio represents a balance between too much reproductive growth (boll production) which can cause the plant growth to outpace the rate of vegetative growth and terminate before the yield is maximized, and too much vegetative growth which reduces the number of mature bolls.
Cotton plants that have directed the majority of the available plant energy to vegetative growth are referred to as "rank" cotton and produce less bolls which mature later and are vulnerable to weather extremes for longer periods of time. Cotton that exhibits signs of going rank are readily visible by abnormal plant height relative to the boll loads and number of reproductive main stem nodes. Mepiquat chloride is used to stop cotton from going rank by modifying the cotton plant's growth characteristics.
The branches off the main stem generally always extend from alternating sides of the stem. Each branch site is called a "node" with 5-7 nodes being formed above the cotyledon leaves before the first fruit bearing branch with true leaves is formed. Node counting starts at the bottom of the plant and extends up the main stem. The "internode length" is the distance between branch sites with a new node being formed roughly every three days. For purposes of measurement and comparison, the number of nodes and internode length above node 8 are generally used to eliminate interplant fruiting node variations because fruit bearing branches will necessarily have been formed by node 8. The counting of fruiting nodes thus conventionally starts from the first reproductive node, usually no. 7 or no. 8.
Fruiting sites in cotton are referred to as "squares." Each fruit bearing branch will form 1-6, normally about 3, fruiting sites ("squares") with approximately six days between square formations on each branch. New squares and the beginning of reproductive growth in cotton plants are referred to as "pinhead" squares due to their barely visible size. After about 5-10 days, the square has grown to about the size of a match head and is a period in the plant cycle referred to as a "match head square." The match head square continues to grow to about the size of an average adult fingernail before blooming ("early bloom"). Three days later, a boll has formed beneath the bloom. Roughly thirty days after early bloom, the product boll is fully mature and ready for harvest. Overall, about 80% of the total cotton yield is set within the first 3 weeks after early bloom and 95% of the total yield is set within 5 weeks of early bloom.
Generally, mepiquat chloride is applied to cotton plants in one of two ways. The method used until about 1986 was a single application of 8-16 ounces per acre of a 4.2 wt % solution at early bloom. This type of single treatments did control plant height although it was noticed that plant yields were occasionally reduced particularly if the plant was stressed during or after the application.
Since 1987, the trend has been to apply mepiquat chloride in a series of applications each having a lower dose than the single dose application. The first treatment occurs at match head square with a second treatment 7-14 days thereafter. Both treatments are made at a rate within the range from about 0-8 ounces of 4.2 wt % solution per acre with the specific application rate depending on whether the cotton plant was exhibiting signs of being stressed (no application), moderate growth (about 2 ounces of solution per acre), or vigorous growth (about 4 ounces of solution per acre). Thereafter, two additional treatments at 7-14 day intervals may be used with application rates extending up to about 8 ounces of 4.2 wt % mepiquat chloride solution with the specific application rate dependent on the amount of vegetative growth in the field. Further experimentation by individual growers has resulted in a wide variety of multiple application rates.
It would be desirable if the use of mepiquat chloride could be integrated into a system of treatment that would increase plant tissue mass in the roots, stems, and leaves to provide higher levels of nutrient transfer while, at the same time, restricting vegetative growth to enhance fruit production.